1. Field of the Invention
The present invention relates to a deposition mask frame assembly, a method of fabricating the same, and a method of fabricating an organic electroluminescent (EL) device using the deposition mask frame assembly, and more, particularly, to a mask frame assembly for depositing a thin film that constitutes an organic EL device.
2. Description of the Related Art
EL devices, which are spontaneous light-emitting display devices, provide a wide viewing angle, a good contrast, and a high response speed. Accordingly, much attention has been recently paid to EL devices because they can be used as next-generation display devices.
EL devices are classified into inorganic EL devices or organic EL devices, depending on what material is used to form a light-emitting layer. Organic EL devices have a higher brightness and faster responsivity than inorganic EL devices, and can provide color display, so they are being actively developed of late.
Such organic EL devices include first electrodes formed in a predetermined pattern on a transparent insulating substrate, an organic film formed on the first electrodes by vacuum deposition, and second electrodes formed as a cathode electrode layer on the organic film such that the first and second electrodes cross each other.
In the manufacture of the organic EL devices having such a structure, the first electrodes are typically formed by patterning indium tin oxide (ITO) using a photolithographic method.
Such a photolithographic method can be used before an organic film is formed, but causes a problem when it is used after an organic film is formed. Because the organic film is very susceptible to damage from water, it must be thoroughly isolated from water while being fabricated, and even after the fabrication. Moreover, the photolithographic method includes an exposure to water during the peeling-off and etching of the resist, so it is not suitable for the organic film and the cathode electrode layer.
This problem is usually solved by vacuum-depositing an organic light emissive material for the organic film, and a material for the cathode electrode layer, using a patterned mask. In particular, the cathode electrode layer can be patterned using a cathode separator, which is a predetermined separation wall, but it is known that a vacuum deposition is the most appropriate method to pattern an organic film formed of a low molecular organic material.
A technique of patterning an organic film, which is a light-emitting layer, using a mask is very important in the manufacture of full-color organic EL devices.
Examples of conventional full-color organic EL device coloring methods include a three-color independent deposition method of independently depositing red (R), green (G), and blue (B) color pixels on a substrate, a color conversion method (CCM) of forming a color conversion layer on a light emissive surface using a blue light source, and a color filtering method which uses a white light source and a color filter. The three-color independent deposition method is very popular because of the simplicity of the procedure, and because it provides a high color purity and efficiency.
In the three-color independent deposition method, a highly accurate mask must be used to independently deposit R, G, and B color pixels on a substrate. In particular, a high accuracy of the positions of the deposited pixels, that is, a high accuracy of the widths of pattern apertures, is required, and a high accuracy of a total mask pitch is also required.
A mask 10 used to deposit an organic film or electrodes during the fabrication of an organic EL device is typically supported by a frame 20, so that the mask 10 is tensed as shown in FIG. 1. The mask 10 is comprised of a single metal thin plate 11 and masking pattern units 12, which allow a plurality of substrate units that constitute an organic EL device to be deposited on the metal thin plate 11.
Because the mask 10 is thinly formed and minutely patterned, some parts of it may droop if it is used without any treatment, preventing an accurate patterning. Accordingly, as shown in FIG. 1, an optimal tension is applied to the mask 10 to obtain a predetermined accuracy of a total pitch (Pt), and the tensed mask 10 is coupled to the mask frame 20. Upon the coupling, it is important to not change the Pt accuracy. The coupling of the mask 10 to the mask frame 20 can be achieved by various methods such as using an adhesive, by laser welding, or by resistance welding.
The mask 10 may be fabricated by etching or electro-forming.
In the manufacture of the mask 10 by etching, a photoresist layer having a pattern of slits is formed on a thin plate, or a film having a pattern of slits is attached to a thin plate, and then the resulting plate is etched.
However, as masks are enlarged and the pattern of the slits becomes fine, the mask forming method based on etching cannot equalize the tolerance of the width of the mask to the tolerance of the edges of the slits. In particular, when the mask 10 is manufactured by etching a thin plate, the thin plate may be over-etched or under-etched. In this case, the sizes of the slits may not be standardized.
On the other hand, the electro-forming method has a principle that a metal is deposited on a prototype to a desired thickness by the electrolysis of a metallic salt solution through an operation such as electrical plating, and then peeled off from the prototype to obtain a metal product having an uneven surface whose prominences and depressions are arranged opposite to those of the prototype. According to this principle, a mask is fabricated.
In the method of fabricating a mask by the electro-forming method, an alloy of nickel (Ni) and cobalt (Co) is used as the material of the mask. The use of the Ni—Co alloy increases the roughness of a fabricated surface and the accuracy of a slot pattern. On the other hand, because the Ni—Co alloy has a poor weldability, a crack may be generated in the mask when the Ni—Co alloy is welded into a frame. In other words, when cobalt is alloyed with another metal, its hardness and intensity increase, and accordingly its fragility increases, which helps a crack to be easily generated upon welding.
This crack generation can be seen from FIGS. 2A through 2C. Referring to FIG. 2A, when the mask 10 and the frame 20 are welded by a laser welder 21, a gap 14 is generated around a joint 13 by heat flexion. If the welding continues on the resultant structure having the gap 14 as shown in FIG. 2B, a crack 15 is generated as shown in FIG. 2C. Due to this crack generation, the tension which supports a mask partially decreases, and accordingly the accuracy of the total pitch of the mask varies. Thus, accurate patterning is impossible.
A mask that overcomes the problem of creeping of strips that form slits due to a thermal expansion of the mask is disclosed in Japanese Patent Publication No. 2001-247961.
The disclosed mask includes a mask portion and a screen portion. The mask portion is a deposition mask used to form a patterning film on a substrate by deposition, and has partitions for defining a plurality of first apertures. The screen portion has a plurality of second apertures smaller than the first apertures, and a magnetic material in which the second apertures are disposed on the first apertures of the mask portion.
Japanese Patent Publication No. 2001-273979 discloses the structure of a magnetic mask. Japanese Patent Publication No. 2001-254169 discloses a deposition mask frame assembly, in which a patterned mask masks a deposition area in a close adherence to a material to be deposited and has finer gaps and finer patterns incapable of supporting a predetermined size than the thickness of a frame. The fine patterns of the patterned mask are supported by fine ribs.
These disclosed masks are closely adhered to a material to be deposited because they are formed of a magnetic material. However, due to the limits of the mask materials, these masks still have fundamental problems, such as a poor welding quality, and a variation in the accuracy of a total pitch due to the poor welding.
Japanese Patent Publication No. 2002-9098 discloses a pattern forming apparatus for preventing a film pre-formed on a substrate from being damaged due to a partial coming-off of a mask from a frame due to a thermal expansion during deposition. The pattern forming apparatus includes a support which is formed to be larger than the mask and has a dent portion to seat the mask onto the dent portion. The user of the support prevents a mask from being bent in waves due to a thermal expansion during the formation of a film. Also, by forming a magnetic element on the side of the mask other than the side on which the support is formed, the magnetic element makes the mask closer to the substrate so that a space between the mask and the support is created. Thus, the space contributes to cooling the mask.
However, because the disclosed mask having slits is not firmly supported by the frame, the location of the mask cannot be accurately controlled, and the location of the mask may be changed during deposition.
Japanese Patent Publication No. 2002-8859 discloses a pattern forming apparatus for preventing a mask from being expanded by heat during the formation of a film, in which a liquid path is formed within a frame which supports the mask, and a cooling solution circulates within the liquid path. However, this disclosure also has a problem such as a change in the accuracy of a total pitch due to a poor welding of a mask and a frame.
Japanese Patent Publication Nos. 2000-48954, 2000-173769, 2001-203079, and 2001-110567 disclose a metal mask including supplementary lines to prevent drooping of a mask shield between the mask and a frame. These masks also have fundamental problems such as a poor welding quality due to the limits of the material of a mask.